In automotive passenger restraint systems (seatbelt systems) it is often necessary to energize a load such as a solenoid or coil for a brief time. For example, seatbelt comfort release systems employ a switching means in the seatbelt buckle to activate a one-shot circuit. This circuit produces an electrical pulse that is used to briefly energize a solenoid coil in a seatbelt retractor assembly, thereby releasing and retracting the seatbelt.
Previously, providing single switched activation of a load has required reliance upon circuits that utilize components such as SCRs and relays. SCRs are edge-triggered devices that switch current to their outputs when small changes in voltage are detected at their inputs. Relays are electromechanical devices typically utilizing a coil to activate mechanical switch contacts.
Known one-shot activation circuits suffer from several defects. First, known switching circuits consume at least moderate amounts of battery current (quiescent draw) even when not in use. This characteristic is particularly disadvantageous for automotive applications where battery current drain must be minimized. In addition, known circuits utilizing SCRs are typically unreliable. SCRs are notoriously sensitive to electrical noise and often activate unnecessarily. Finally, circuits using electromechanical relays require more space and improved shielding because relays are relatively large and electrically noisy devices.